DESCRIPTION Individual olfactory sensory neurons express one or a few odorant receptor isoforms from a repertoire of nearly a thousand. This regulated odorant- receptor gene expression defines the phenotype of a olfactory sensory neuron and is critical to information processing in the olfactory system. The experiments proposed here will test the hypothesis that the decision to express a particular odorant receptor occurs largely in neuronal precursor cells. Neurons descendent from the same precursor will be identified by labeling individual cell genealogies through replication-incompetent retroviral gene transfer. The odorant receptor expression patter of neurons within the same genealogy will be investigated in two ways: 1) Related neurons expressing identical odorant receptors will be identified by analyzing their projection patterns. This experiment relies on the assumption that all olfactory sensory neurons expressing the same odorant receptor project to the same one or two glomeruli. 2) The odorant receptor isoform(s) expressed by related neurons will be determined by cloning odorant receptor cDNA from single-single cDNA libraries. By comparing the odorant receptors expressed by neurons in the same genealogy we will determine whether neurons arising from the same basal cell express the same or closely related odorant receptors, or whether they express different and unrelated odorant receptors.